A general requirement for any rate-responsive pacemaker is a sensor for detecting a physiological parameter which varies with the body's metabolic demand for cardiac output of blood. Preferably, a rate-responsive pacemaker will monitor a physiological parameter which accurately responds to physical and emotional stimulation wherein changes in the parameter and in metabolic demand vary in a linear fashion. Various types of rate-responsive pacemakers have been developed which provide different approaches to metabolic-demand sensing. These pacemakers may measure different physiological parameters or measure a particular physiological parameter in a different manner to provide a basis for rate-adaptive pacing. Each of these different approaches to metabolic-demand sensing may be advantageous or disadvantageous for a particular patient or cardiac malfunction.
The present invention provides a rate-adaptive sensor within a pacemaker which allows the pacemaker to automatically match the pacing rate to the patient's metabolic demand and to respond quickly to changes in the metabolic demand. The operation of the sensor may be altered by means of programming of the pacemaker from an external communicating device. These alterations in sensor operation fulfill the needs of various patients who are afflicted with different cardiac and respiratory health problems. This sensor does not require special leads or special sensor transducers other than those common in standard cardiac pacemakers.
One common metabolic-demand sensor measures physical activity to provide a suitable parameter for rate adaptation. A physical activity sensor is not generally regarded as a truly physiologic sensor because it does not measure true metabolic demand and, therefore, is not affected by emotional stimuli or pyrexia. In U.S. Pat. No. 4,140,132, entitled "Variable Rate Timer for a Cardiac Pacemaker", issued to J. D. Dahl on Feb. 20, 1979, a pacemaker employs an accelerometer, an implanted weighted cantilever arm piezoelectric crystal, to monitor the physical activity of a patient and set the pacemaker's escape interval. Similarly, in U.S. Pat. No. 4,428,378, entitled "Rate Adaptive Pacer", issued on Jan. 31, 1984, K. M. Anderson et al. describe a sensor which generates a signal reflecting the activity of a patient. The pacemaker bandpass filters this signal, detects its amplitude and derives a pacing rate from the processed signal. The high frequency content of this signal increases with patient movement, therefore the pacemaker modulates the pacing rate, between preset rate maxima and minima, in proportion to the intensity of the processed signal.
The most important advantage of a physical-activity sensor is its very rapid response time to the onset of exercise. A physical-activity sensor responds favorably to patient activities which create vibration, such as jogging, walking and stair climbing. Unfortunately, activities such as bicycling do not promote rate adaptation because little vibration occurs.
Further advantages of a physical-activity sensor lie in its simplicity. No special pacing lead is required since an activity-based pacemaker may employ standard leads with either unipolar or bipolar electrodes. Furthermore, no special implanting procedure is required for an activity-based pacemaker.
Although the lack of a truly physiological response is generally considered a disadvantage of an activity sensor, the fact that this sensor acts independently from physiologic variables may provide a better response under conditions in which patient systems or tissues are diseased. For example, an activity sensor may supply a better signal for responding to exercise than a respiration sensor will under conditions of lung disease, such as emphysema.
The primary disadvantage of a rate-responsive pacemaker employing a physical-activity sensor is the difficulty of attaining a scaled response to gradations of metabolic demand. Activity sensors generally act in an on/off fashion, in which a sensor is unable to detect changes in patient workload. Therefore, the response of activity-based, rate-responsive pacemakers does not normally depend on the amount of exercise the patient is performing, but instead the rate change remains identical so long as the measured activity is above a preprogrammed level. Because it is difficult or impossible to relate the amount of vibration of the sensor to the cardiac output needs of a patient performing activity of various types, the sensor cannot be programmed to adapt the pacing rate in a physiological manner. In particular, it is difficult to program the pacemaker to correctly respond to the onset or cessation of exercise.
Furthermore, a physical-activity sensor generates an undesirable response to noise disturbances arising external to the body (e.g., machinery) or from within the body (e.g., coughing, sneezing and laughing). Also, noise signals tend to swamp activity-induced signals which occur at some frequencies.
A second type of metabolic-demand sensor measures and analyzes impedance signals which relate to cardiac mechanical performance to adapt the pacing rate to the metabolic demands of the patient. Pacemakers analyze and process cardiac mechanical data to derive physiological parameters such as stroke volume or cardiac output. For example, a pacemaker may utilize an intravascular-impedance sensor to measure right ventricular stroke volume and adjust pacing rate to keep this parameter at predetermined physiologic values. In U.S. Pat. No. 4,535,774, entitled "Stroke Volume Controlled Pacer", issued on Aug. 20, 1985, W. H. Olson describes an impedance plethysmography sensor, comprising a number of electrodes and analysis circuitry, which is employed to detect variations in stroke volume over time. The pacemaker sets the pacing rate according to these changes in stroke volume.
The primary advantage of using stroke volume as a parameter for adjusting rate in a rate-responsive pacemaker is the capability of rapidly adjusting the rate to changes in metabolic demand in a physiologic manner.
The main disadvantage of the stroke-volume, rate-responsive pacemaker is its requirement for a nonstandard pacing lead having multiple electrodes. Sensing of impedance using standard bipolar leads has not provided the accuracy in the stroke volume measurement which is necessary for rate-adaptive control. Preferable tripolar or quadripolar leads have not been durable enough for chronically implanted usage. Furthermore, an appropriate algorithm for driving a closed loop adaptive pacing rate has not been discovered. Rate-response algorithms using stroke volume as a control parameter have been most ineffective for sick patients. A disadvantage of stroke-volume controlled rate adaptation, in comparison to rate control based on physical activity, is its requirement for complex sensors and circuits and an inability to use standard pacing leads.
Another type of metabolic-demand sensor measures and analyzes impedance signals which relate to a patient's respiratory function to adjust pacing according to the metabolic demands of the patient.
In U.S. Pat. No. 4,567,892, entitled "Implantable Cardiac Pacemaker", issued to G. Plicchi and G. Canducci on Feb. 4, 1986, a pacemaker is disclosed which monitors respiratory rate by measuring impedance variations throughout a distance within the thoracic region of a patient's body, between the pacemaker can and a separate auxiliary or passive lead implanted subcutaneously in the chest wall using a special tunneler. A programmed algorithm within the pacemaker analyzes the respiratory rate to determine a pacing rate. It has been shown that heart rate, respiratory rate and oxygen uptake all correlate well irrespective of the presence of lung disease. All parameters increase at the onset of exercise and decrease when exercise stops. The rate-responsive pacemaker which is driven by the respiration rate measurement is simple and reliable, as well as sound in its physiologic basis.
A fundamental disadvantage of driving the pacing rate on the basis of respiratory-rate variations is that it takes into account only part of the body's ventilation adaptation in response to exercise. Ventilation increases due to an increase in the depth of respiration as well as the respiration rate. Although respiratory rate relates somewhat closely to heart rate, heart rate correlates much more strongly with the total amount of inspired air. That the Plicchi and Canducci pacemaker requires a special surgical procedure, tunneling of a lead in the patient's thoracic region, and a special sensor, are practical disadvantages of the device.
M. S. Lampadius, in U.S. Pat. No. 4,721,110, entitled "Respiration-controlled Cardiac Pacemaker", issued on Jan. 26, 1988, improves on the respiration-rate-driven pacemaker by disclosing a pacemaker driven either by respiration depth or respiration rate. This pacemaker employs a rheography pulse generator, which generates constant amplitude pulses during the refractory period of a patient's heart, and a respiration detector which, as a function of the impedance data measured in response to the rheography pulses, generates a respiration signal representing the respiratory rate, the depth of respiration or a combination of the rate and depth of respiration. The pacemaker then uses this respiration signal to determine an appropriate pacing stimulation rate.
The respiratory parameter which correlates most closely to heart rate is minute ventilation, a highly physiologic variable which reflects closely the metabolic demands of exercise. The body's increase in minute ventilation during exercise parallels its oxygen uptake but also reflects changes in cardiac output and heart rate. Minute ventilation not only correlates well with exercise, but also varies in response to stress and pyrexia. U.S. Pat. No. 4,702,253 (hereinafter called the "'253 patent"), entitled "Metabolic-Demand Pacemaker and Method of Using the Same to Determine Minute Volume", issued to T. A. Nappholz et al. on Oct. 27, 1987, discloses a rate-responsive pacemaker which senses impedance in the pleural Cavity of a patient and derives respiratory minute volume from impedance. The pacemaker then employs the respiratory minute volume, a measure of the amount of air inspired by a person as a function of time, as a rate-control parameter. The greater the amount of air inspired, the greater the need for a higher pacing rate. The device described in this patent requires a nonstandard pacing lead in order to perform the minute volume measurement.
U.S. Pat. No. 4,901,725 (hereinafter called the "'725 patent"), entitled "Minute Volume Rate-Responsive Pacemaker", issued to T. A. Nappholz et al. on Feb. 20, 1990, discloses a pacemaker which performs a rate-responsive function in the manner of the '253 patent with various improvements, and, in addition, only requires standard pacing leads. To measure the intravascular impedance, the minute-volume sensor generates a low energy current pulse at 50 ms intervals between a ring electrode of the lead and the pulse generator case, then measures the voltage between the tip electrode of the lead and the pulse generator case arising from the applied current. An intravascular impedance value is determined from the measured voltage and the applied current using Ohm's law. Transthoracic impedance increases with inspiration, decreases with expiration and its amplitude varies with the tidal volume. The impedance signal thus comprises two components, representing tidal volume and respiratory rate. Pulse generator circuitry identifies the two signals and processes them to yield minute ventilation. The minute-volume-controlled rate-responsive pacemaker employs a highly physiologic sensor. Its ability to assess the metabolic demands of the body are superior to that of a pacemaker driven by respiratory rate alone, since depth of ventilation is an important response to exercise or stress. The apparatus described in the '725 patent requires no more than standard pacing leads, although the leads cannot be unipolar leads, and programming of minute ventilation rate adaptation necessitates only a single exercise test.
Pacemakers which use any of the discussed respiratory parameters as a basis for rate adaptation are considered to respond more slowly to the onset of exercise than a physical activity controlled pacemaker. A faster response is more desirable.
An advantage of the activity-sensing pacemakers is its fast response to the onset of exercise, but a major disadvantage of a pacemaker which determines pacing rate based on an activity signal alone is its substantial inability to react to the instantaneous metabolic level of exercise or stress. A pacemaker which uses a more physiologic parameter may respond less quickly to the onset of exercise, but is highly specific with respect to the metabolic level of exercise. Thus, a pacemaker may employ two parameters in combination in a rate adaptive cardiac pacing system, wherein each parameter complements the other by mutually supplying what the other lacks.
U.S. Pat. No. 4,926,863, entitled "Rate-responsive Cardiac Pacemaker", issued to E. Alt on May 22, 1990, discloses a rate-responsive cardiac pacemaker which employs an activity sensor, in the form of an accelerometer, as a first sensor of metabolic demand and a second sensor which is adapted to detect a parameter which is complementary to activity. The measurement of the second sensor is used to supply a "complementary parameter" for confirming the presence of a particular metabolic state and selectively contributing to the determination of a stimulation rate. The complementary parameter of the second sensor is defined as any physiological or other detected parameter, within or outside the body, having characteristics of sensitivity and specificity to physical exercise which contrast and enhance the corresponding characteristics of the activity sensor, specifically, a fast response time to the onset of exercise but nonspecificity with respect to the instantaneous metabolic level of exercise. The Alt patent mentions the parameter of central venous blood temperature as a possible complementary parameter.
U.S. Pat. No. 4,860,751, entitled "Activity Sensor for Pacemaker Control", issued to F. J. Callaghan on Aug. 29, 1989, also discloses a rate-responsive cardiac pacemaker which includes an activity sensor in combination with a second sensor for monitoring a physiological parameter such as partial pressure of oxygen (pO.sub.2), blood pressure, core temperature, CO.sub.2 and pCO.sub.2, O.sub.2 and pO.sub.2, pH, respiration rate, respiration depth and ventricular volume. In this patent, the activity sensor provides for generally constant monitoring of patient motion. When patient motion causes the activity sensor to generate a signal exceeding a preset threshold level, the physiological sensor is activated. The parameter generated by the physiological sensor is used to set pacing rate. The advantage of using an activity sensor as a trigger to initiate sensing by a physiological sensor is that the activity sensor requires very little energy expenditure to operate. In contrast, a physiological parameter sensor normally consumes a great deal of energy.